In a conventional separated cooling system for an engine, as shown in FIG. 2, individual coolant inlet lines 10a and 11a introduce coolant into cylinder head 10 and cylinder block 11, and individual coolant outlet lines 10b and 11b discharge coolant from cylinder head 10 and cylinder block 11. Individual coolant inlet lines 10a and 11a and individual coolant outlet lines 10b and 11b are formed separately. U.S. Pat. No. 6,595,164 discloses one example of a separated cooling system for an engine.
In the conventional separated cooling system, coolant pump 12 for supplying coolant to cylinder head 10 and cylinder block 11, and radiator 14 for radiating the heat of coolant discharged from cylinder head 10 and cylinder block 11, are commonly used for cooling cylinder head 10 and cylinder block 11, but individual coolant inlet lines 10a and 11a and individual coolant outlet lines 10b and 11b are formed separately.
Cylinder-head-side coolant outlet line 10b connects cylinder head 10 and radiator 14 such that coolant discharged from cylinder head 10 flows directly into radiator 14, and a coolant line is formed such that coolant, having passed through radiator 14, flows to coolant pump 12. Main thermostat 13 is disposed in a coolant line connecting radiator 14 and coolant pump 12. Main thermostat 13 controls the flow of the coolant such that the coolant is supplied to cylinder head 10 and cylinder block 11 via coolant pump 12 only when the temperature of the coolant is within a predetermined temperature range. A portion of the coolant discharged from cylinder head 10 is supplied to heater 18 for heating a passenger room, and then is supplied to coolant pump 12 via main thermostat 13.
Meanwhile, block thermostat 15 for controlling flow of coolant discharged from cylinder block 11 is disposed in cylinder-block-side coolant outlet line 11b, and coolant, having passed through block thermostat 15, is supplied to coolant pump 12 via radiator 14. A portion of coolant discharged from cylinder-block-side coolant outlet line 11b is used for cooling oil cooler 16. A bypass line connecting block thermostat 15 and coolant pump 12 is formed. When the temperature of coolant discharged from coolant outlet line 11b is lower than a predetermined temperature, block thermostat 15 closes the coolant line so that coolant discharged from coolant outlet line 11b is directly supplied to coolant pump 12 via the bypass line.
In such a conventional cooling system, in which the coolant passages for cylinder head 10 and cylinder block 11 are separately provided, there is a disadvantage in that the pressures of the coolant supplied to cylinder head 10 and cylinder block 11 become non-uniform. Additionally, when block thermostat 15 is closed, the flow of the coolant in cylinder block 11 becomes congested. This causes the heat grade to deteriorate, and cylinder block 11 may become damaged by the heat.